This invention relates generally to a solder paste mixture which is easily detectable and more particularly to a solder paste mixture which fluoresces under ultraviolet illumination.
Solder pastes comprising solder fragments dispersed in a solder flux are commonly used in the assembly of circuit boards, particularly those using leadless components. Small quantities of solder paste are applied to metallic solder pads on the circuit board, electrical components are placed on the circuit board such that an electrical contact means contacts the solder paste, and the solder paste is heated to fuse the solder, often by passing the circuit board bearing the solder paste and components through an oven. The circuit board is then allowed to cool so that the solder solidifies, bonding and electrically connecting the electrical components to the solder pads. The surface of the circuit board is then washed to remove the solder flux component of the solder paste which may lead to corrosion if allowed to remain on the completed circuit board.
With automation of the solder paste application, component application, solder fusion and washing operations, automatic inspection of the circuit board at particular stages of its assembly may be desirable. For example, inspection of the circuit board immediately following solder paste application verifies that solder paste has been applied to appropriate locations on the circuit board; inspection of the circuit board immediately following component placement verifies that components have been placed in appropriate locations on the circuit board; and inspection of the circuit board assembly following washing verifies that the solder flux has been removed, and that solder and components are fixed to appropriate locations on the circuit board.
When a fluorescent pigment is dispersed in the solder paste, the solder paste will fluoresce under ultraviolet illumination. The fluorescence increases the contrast between the solder paste and the circuit board or components, facilitating automatic detection of the solder paste. The increased contrast between the solder paste and the components also facilitates automatic detection of component edges which overlap the solder paste. Moreover, if a portion of the pigment disperses in the solder when the solder paste is heated to the melting point of the solder, the pigment will also facilitate automatic detection of the solder following the solder fusion and washing operations. In this case, the contrast will further aid in automatic detection of component edges which overlap the solidifed solder.
Thus, dispersion of a fluorescent pigment in solder paste facilitates automatic inspection of the circuit board for verifying the location of solder paste, verifying the placement of components and verifying the placement of fused and solidified solder. The fluorescent pigment also facilitates automatic detection of solder paste during the component placement operation, when such detection could be used to aid proper placement of the components, and could facilitate automatic detection of solder during solder joint integrity testing.
U.S. Pat. No. 3,139,360 discloses an inspectable soldering flux composition whose presence is detectable in trace amounts in ultraviolet light. This soldering flux composition incorporates an ultraviolet absorbing material which renders the flux readily visible under ultraviolet illumination. The ultraviolet absorbing material is removed with the flux when the flux is washed from a printed circuit board following fusion of the solder. Consequently this solder flux composition aids in the detection of unwanted solder flux residue on printed circuit boards following washing.
U.S. Pat. No. 3,406,285 discloses a method of inspection of soldered joints comprising using a volatile fluorescent dye which, upon heating during soldering to a satisfactory soldering temperature range, will no longer fluoresce during a subsequent inspection. The dye is applied before the soldering operation to the components (such as wires and terminals) to be soldered. If the components are heated to a satisfactory soldering temperature during the soldering operation, no fluorescence will be detected when the components are inspected under ultraviolet illumination. However, if the components do not reach a satisfactory soldering temperature, the dye will fluoresce when the components are inspected under ultraviolet illumination. Consequently, the dye may be used to detect suspect solder joints on printed circuit boards.
The present invention is directed to facilitating the detection of solder paste, fused and solidified solder, and component edges which overlap the solder paste or solder. The methods disclosed in U.S. Pat. Nos. 3,139,360 and 3,406,285 do not address this problem. The materials these patents describe would not be effective in solving the problem addressed by the present invention. The ultraviolet absorbing material of U.S. Pat. No. 3,139,360 would not disperse in the solder during the fusion operation and would be removed from the circuit board with the flux during the washing operation. The volatile dye of U.S. Pat. No. 3,406,285 would lose its fluorescent properties during the fusion operation.
Accordingly, it is an object of this invention to provide a solder paste mixture which fluoresces under ultraviolet illumination and which, after fusion of the solder and removal of the solder flux, yields a solder which fluoresces under ultraviolet illumination.
It is also an object of this invention to provide a method for making this solder paste mixture.
It is another object of this invention to provide a method for making a fluorescent solder joint between an electrical component and a circuit board.